Implement autoplay gesture override experiment.

LayoutTests/media/video-autoplay-experiment-expected.txt

Index: LayoutTests/media/video-autoplay-experiment-expected.txt
diff --git a/LayoutTests/media/video-autoplay-experiment-expected.txt b/LayoutTests/media/video-autoplay-experiment-expected.txt
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+CONSOLE WARNING: Failed to execute 'play' on 'HTMLMediaElement': API can only be initiated by a user gesture.
+CONSOLE WARNING: Failed to execute 'play' on 'HTMLMediaElement': API can only be initiated by a user gesture.
+CONSOLE WARNING: Failed to execute 'play' on 'HTMLMediaElement': API can only be initiated by a user gesture.
+CONSOLE WARNING: Failed to execute 'play' on 'HTMLMediaElement': API can only be initiated by a user gesture.
+CONSOLE WARNING: Failed to execute 'play' on 'HTMLMediaElement': API can only be initiated by a user gesture.
+END OF TEST
+  Check if the autoplay gesture override experiment works.  There are a lot
+  of config options, so this test just runs all of them.
+
+  The "results" table contains one row per config tested.
+  == Test Inputs ==
+  #      - config number, in case you'd like to run just one.
+  Exp    - autoplay experiment setting being tested.
+  A-play - how is autoplay requested?
+               none - autoplay is not requested.
+               attr - autoplay attribute is set on the element.
+               js   - play() called after media is ready to play.
+  Mute   - how is media muted?
+               none - media is not muted.
+               attr - muted attribute is set on the element.
+               js   - muted property is set after media is ready to play.
+  Vis    - is media visible?
+               onscreen  - element starts out onscreen.
+               scroll    - element starts offscreen, scrolled into view once
+                           it is ready to play.
+               offscreen - element starts out offscreen and stays offscreen.
+
+  == Test Outputs ==
+  Early? - did playback start before element was scrolled onscreen?  For
+           tests in which Vis!=scroll, this is reported as "-".
+  Played? - did playback start by the conclusion of the test?
+  Muted? - was the media muted?  If the media didn't play, then this is
+           reported as "-".
+
+#	Exp	A-play	Mute	Vis	Early?	Played?	Muted?
+0	none	none	none	onscree	-	no	-
+1	always	none	none	onscree	-	no	-
+2	if-mute	none	none	onscree	-	no	-
+3	play-mu	none	none	onscree	-	no	-
+4	none	attr	none	onscree	-	no	-
+5	always	attr	none	onscree	-	played	unmuted
+6	if-mute	attr	none	onscree	-	no	-
+7	play-mu	attr	none	onscree	-	played	muted
+8	none	js	none	onscree	-	no	-
+9	always	js	none	onscree	-	played	unmuted
+10	if-mute	js	none	onscree	-	no	-
+11	play-mu	js	none	onscree	-	played	muted
+14	if-mute	none	attr	onscree	-	no	-
+18	if-mute	attr	attr	onscree	-	played	muted
+22	if-mute	js	attr	onscree	-	played	muted
+26	if-mute	none	js	onscree	-	no	-
+30	if-mute	attr	js	onscree	-	played	muted
+34	if-mute	js	js	onscree	-	played	muted
+40	none	attr	none	scroll	no	no	-
+41	always	attr	none	scroll	no	played	unmuted
+43	play-mu	attr	none	scroll	no	played	muted
+44	none	js	none	scroll	no	no	-
+45	always	js	none	scroll	no	played	unmuted
+47	play-mu	js	none	scroll	no	played	muted
+76	none	attr	none	offscre	-	no	-
+77	always	attr	none	offscre	-	no	-
+79	play-mu	attr	none	offscre	-	no	-
+